This is absolutely correct. Most residential mortgages are secured by GSE's such as Fannie Mae, Freddie Mac, VA, USDA, or FHA. Let's say a homeowner gets a mortgage for $100K on a $125K property, value drops to $80 and owner walks for whatever reason, bank forecloses, and buy it at sheriff auction for $50. They are in the hole for $100K (what was owed), not what they spent at the sheriff sale. In order to receive the $100K from one of the GSE's, they have to market it according to the terms of the GSE. Fannie is different than Freddie, VA is different than USDA, etc.
I guess the point I am trying to make is that is isn't necessarily the banks fault. It's also not the banks fault the property is in foreclosure. I cringe when I hear that a homeowner gives a property back to a bank. The homeowners don't "give it back" to the bank, they give it to them instead of paying back the money the bank gave them to start with.
I do not own or operate a bank...I do work wth foreclosure properties, both for banks and for property owners.